1. Field of the Invention
This invention relates to a device and to a method. More specifically, this invention is directed to a self-supporting flexible airfoil which isresistant to possible damage by relatively strong or gusty winds. This invention further provides a method for reinforcement of an airfoil by simple and efficient means without increasing the weight thereof.
2. Description of the Prior Art
The design of an airfoil involves certain aerodynamic principles which govern its lift, flight and stall characteristics. The airfoil can either be relatively inflexible, such as in the case as a fixed wing aircraft, or relatively flexible, as in the case of the design of certain kites. In each configuration, the lift on the airfoil is created by an air pressure differential created by the relative wind as it flows over the top and underside of the airfoil. This differential can be altered and the airfoil stalled where the angle of attack of the airfoil is increased beyond permissible limits. A second technique for reducing the lift on an airfoil is to alter the shape of the airfoil itself. In a typical aircraft wing, a spoiler can be deployed, much in the same way as flaps, and thus increase the rate of descent of an aircraft (without otherwise altering the angle of attack or air speed of the aircraft). The spoiler is typically activated prior to landing.
In the design and manufacture of flexible airfoils, a number of alternative configurations have been suggested. Each such alternative generally has one or more inherent limitations either with respect to its structural integrity and/or ability to accommodate strong and/or gusty winds. The prior art relating to kite construction is representative of such flexible airfoil construction and is, thus, discussed hereinafter in some detail.
With respect to such kites, there are basically two (2) types of configuration disclosed in the prior art; namely, the more traditional structure incorporating struts or some form of stiffening member and those kites in which the structural integrity and shape of the airfoil is maintained by the inherent stiffness or rigidity of the construction materials themselves. The following list of patents is representative of each type of construction: U.S. Pat. No. 3,752,423 (to Schaeffer); U.S. Pat. No. 4,103,850 (to Bushell); U.S. Pat. No. 2,820,320 (to Levicy); U.S. Pat. No. 4,216,929 (to Holland); U.S. Pat. No. 4,354,647 (to Carpenter); U.S. Pat. No. 3,697,023 (to Jackson); and U.S. Pat. No. 3,507,464 (to Rogallo).
U.S. Pat. No. 3,752,423 (to Schaeffer) describes a simple and inexpensive technique for forming a kite from one (1), or preferably two (2), rectangular sheets of pliable and relatively stiff paper or plastic construction materials. The Schaeffer kite is formed by simply folding a rectangular sheet of construction materials along the centerline, which is to later define the location of the keel of such kite. Each of the two (2) lateral symmetrical panels to either side of the centerline are now curved, or rolled downward, and fastened to the keel to form a pair of what the inventor characterizes as "wings". The forward corner of the leading edge of each of these wings are fastened to the keel with a staple or other conventional fastening means. The resultant product is reported capable of flight in light to moderate breezes and does not, due to its construction and center of gravity, require a tail. The absence of an struts, spars or other traditional form of stiffening member apparently constrains the use of the Schaeffer kite to relatively gentle wind conditions.
U.S. Pat. No. 4,103,850 (to Bushell) describes the fabrication of a kite from two (2) substantially triangular panels joined together along a first seam, which forms the base of a keel. These triangular panels are also joined together along a second seam (which is parallel to and above the first seam) thereby forming what the inventor characterizes as a "double keel". A stiffening member, or mast, is disposed longitudinally in the lower edge of the keel and thereby maintains the dimensional stability of the keel's shape. The leading edges of each triangular panel are also reinforced, or stiffened, with a spar and a cross member attached to each spar member at right angles to the mast. The foregoing design reportedly improves flight stability in high winds.
U.S. Pat. No. 2,820,320 (to Levicy) describes the fabrication of a gliding or miniature kite from construction materials and stay strips (integral framing members). These stays reinforce and maintain the shape of the kite and can be bent, as desired, to permit selective adjustments of the wing and tail configuration portions of the kite. This kite design is adapted for use with a short tether which is attached to a stick.
U.S. Pat. No. 4,216,929 (to Holland) describes what appears to be an improvement in the Bushell design (U.S. Pat. No. 4,103,850--previously discussed herein). The Holland design enhances the stability of the Bushell kite in relatively strong winds by providing a light weight, dimensionally rigid, reinforcing spar along the entire leading edge of the kite sail. Similar structural rigidity is added to the base of the keel. The differences between the Holland and the Bushell designed are illustrated in their patents at FIGS. 10 and 3, respectively.
U.S. Pat. No. 4,354,647 (to Carpenter) describes a method of fabricating a miniature kite which can be readily assembled and disassembled for ease of storage. Carpenter reinforces the wing section of his kite by providing a "U-shaped" slit (reference numeral 16) in the wing of the kite. When the portion of the wing section defined by slit (16) is folded up, it forms a brace or reinforcing rib (reference numeral 17) which reportedly lends dimensional rigidity to the wing section.
U.S. Pat. No. 3,697,023 (to Jackson) describes the fabrication of the wing control kite having one or more tabs arranged along the outer regions of the leading edge of the kite sail. These tabs, which are disposed in a vertical attitude during flight, prevent the lateral portions of the wings from collapse during rapid wind shift or slight yaw of the kite. Battens are provided to stiffen these tabs relative to the leading edge so as to maintain their configuration during flight. In the preferred flight configuration, these tabs are maintained at an angle to the wing of between fourteen degrees (14.degree.) to sixteen degrees (16.degree.).
U.S. Pat. No. 3,507,464 (to Rogallo) describes a method for fabrication of a flexible wing having reinforcement in the trailing edge of such wing. The relative position of such reinforcing members can be altered and the wing configuration thereby changed. Rogallo indicates that by altering these reinforcing members, increase control of the flexible wings is obtained by "spoiling" the aerodynamic flow over the wing membrane.
As is evident from the foregoing description of the representative prior art, simplicity of airfoil construction is not without sacrifice in flight stability and durability of the resultant structure. Where such reinforcement is added, the additional weight is not without penalty. As discussed in the Holland patent, the addition of reinforcement generally increases the mass of the control surfaces with a reported corresponding improvement in performance in gusty or relatively strong winds. Holland addresses this problem of flight stability by adding structural rigidity to the leading edge of the sail portion of the airfoil. By way of contrast, the Rogallo patent addresses the problem of airfoil stability by modification of the trailing edge of the air flow. In each instance, such improved stability in strong and gusty winds has required the addition of spars, or the equivalent, to the sail portion of the kite. Not only does such reinforcement increase the weight but also the cost of its manufacture. As is thus evident, there exists a continuing need for improvement in kite design wherein the aerodynamic performance of the sail portion of the kite is maintained, stability enhanced, and yet the structural integrity preserved in both gusty and relatively strong winds.